Blimey, creeks and waterways are seriously underrated when it comes to mystery animal sightings. Most people focus on forests and fields but water is where a lot of the really odd stuff happens.
What size are we talking roughly? Like dog-sized or bigger? And did it make any sound or was it just moving silently? I ask because I've been reading a fair bit about large unidentified aquatic animals in British waterways and the descriptions vary wildly.
Where abouts are you located as well - no need to be specific if you dont want but even just the region helps narrow things down. Giant catfish get reported quite a bit in European rivers and some of them get absolutely enormous, but if this thing had legs or was moving in a strange way that changes things completely.
Would love to hear more details. If you get the chance go back in daylight and check the mud along the bank for prints or any disturbance in the vegetation, that kind of physical evidence is gold.